The medical electrode transfers the energy of ionic currents in the body into electrical currents that can be amplified, studied, and used to help make diagnoses.
Medical electrodes permit surface quantification of internal ionic currents, yielding an ordinarily non-invasive test for a variety of nervous, muscular, ocular, cardiac, and other disorders that might otherwise have required surgical means to verify their presence. For instance, muscular exams using electrodes may produce evidence of diminished muscle strength and can discriminate between primary muscle disorders and neurologically-based disorders, in addition to detecting if a muscle is truly weak or seems so due to other reasons. The electrodes are typically easy to use, fairly cheap, disposable (or easily sterilizable), and often unique in the tasks they help to perform. The essential role of the electrode is to provide ideal electrical contact between the patient and the apparatus used to measure or record activity.
Medical electrodes are generally comprised of a lead (for conduction of electrical current), a metal electrode, and electrode-conducting paste or gel for surface electrodes. There is also often a metal (for good electrical contact) snap for the lead to snap into place so that the electrode can be disposable while the lead can be reused.
Electrodes can be classified into many groupings; those useful for EEG, for example, follow:
- disposable electrodes (both types, without gel and pregelled)
- reusable disc electrodes (gold, silver, stainless steel, or tin)
- saline-based electrodes, which include various kinds
Electromyography requires more specialized needle electrodes that must be capable of piercing the skin.
Electrodes are used for diverse procedures and check-ups in the health setting. Combined with monitoring systems, they can be powerful predictors of disease and disorders. Some of the main types of exams utilizing medical electrodes include:
- Electrocardiography (ECG/EKG): Evaluates the electrical activity of the heart. It may be used to assess heart rate and regularity, in addition to damage, effects of drugs, and devices. ECG is also popularly used to determine the size and position of the chambers of the heart as they relate to the onset of various forms of heart disease. Diagnostic ECG may require 12 to 15 surface electrodes, while monitoring ECG usually employs three to five.
- Electroencephalography (EEG): Helps to identify specific irregularities in the brain. Brain wave patterns can be recorded and observed by positioning typically 10 to 20 electrodes upon the scalp of the patient in different areas and measuring ionic, electrical waves of neuronal activity.
- Electromyography (EMG): Assesses muscular response to electrical activity in innervated muscle. Utilizes needle electrodes that are inserted through the skin into relevant muscle fibers.
- Electonystagmography (ENG): Records eye movements graphically by placing metal electrodes above, below, and to the side of the appropriate eye, in addition to a ground electrode on the forehead. Eye motion is then recorded relative to the ground electrode location. Testing is usually performed to verify the presence of nystagmus.
- Electroretinography (ERG): Operates with an electrode positioned onto the cornea of the eye to record the electrical response of retinal rods and cones. Electrodes measure retinal electrical response to the impingement of light in order to assess a probable retinal disorder (both hereditary and acquired) and whether it may require surgery.
Offshoot technologies of electrodes, as of July 2001, are veering toward multi-functional processes. One original electrode application boasts hands-free defibrillator capabilities in addition to its normal electrode functions in ECG. It should also be noted that, using a voltage or current generator, electrical stimulation can be applied to precise areas of the body using medical electrodes (in addition to their more conventional utility in measuring ionic currents).
Prior to performing EMG, ENG, and ERG tests, adults need not take any special precautions, except to inform their medical provider of any medications they take. EEG patients should thoroughly wash their hair the night before and use nothing in their hair (such as hair spray, lotions, or oils) on the day of the test. Discontinuance of medications may be necessary and patients should avoid caffeine for at least eight hours prior to the test. ECG patients should inform the provider of any medications taken, in addition to refraining from ingesting cold water and from exercising immediately before the test. Removal of all jewelry is also required.
Since the role of medical electrodes is generally for monitoring of electrical impulses, there is no risk of shock. Electrical stimulation using electrodes carries more risk because electricity is delivered to the body and should thus only be performed by personnel with an understanding of the risks involved (especially electrical) and how to avoid them. Knowledge of the ground electrodes and how to use them is strictly required and differs for different systems.
Maintenance of electrodes, if they are not disposable, involves sterilization and checking to ensure that the electrode is electrically viable by following the manufacturer's instructions.
Health care team roles
Doctors, nurses, or other technicians may need to perform tests involving medical electrodes. Often the technician or nurse applies the electrodes in patterns conforming to testing standards (i.e., Einthoven's triangle).
Electrode placement is essential and thus must be known well enough to position electrodes correctly to obtain viable data on many different types of patients. Since each procedure is unique, training must be appropriate to the specific procedure being performed. Electrode impedances may be checked to ensure superior electrical contact; the optimal values should be listed in the manufacturer's manual.
In using electrodes, the technician should follow the guidelines set in the manufacturer's manual, because electrode handling is not conserved over all electrode types and applications. The following general guidelines have been adapted from Biomedical Life Systems (as of July 2001), a major manufacturer of medical electrodes, but do not apply to all types of electrodes:
- Electrodes, gel, and tape (for securing the electrode) should not be applied over broken skin.
- Body hair should be trimmed or shaved where electrodes are going to be applied.
- Carbon electrodes should be cleaned with mild soap and water to remove skin oils, gel, and dirt.
- Skin should be cleaned before and after applying electrodes.
- Cleaning lead wires with mild soap and water will prevent them from becoming brittle.
Einthoven's triangle—An ECG reference system with the subject's heart presumed to be the center of an equilateral triangle (both physically, in that electrodes are placed in form of a triangle on the body, and figuratively, in the mathematical measurement techniques used to assess a patient's cardiac health) formed by three bipolar limb leads. Can be used to diagnose various disorders of the heart. Named after Wilhelm Einthoven, the Dutch electrocardiographer who advanced these recording techniques.
Nystagmus—Rapid, repetitive involuntary movements of the eyes.
Retinal rods and cones—The two types of light-sensitive photoreceptor cells in the retina.
Saline—Containing a salt, or of the nature of a salt.
Mroz, A., M. Borchardt, C. Diekmann, K. Cammann, M. Knoll, and C. Dumchat. "Disposable Reference Electrode." The Analyst 123, no. 6. (June 1998): 1373–6.
Medical Electrodes (ECG, EEG, EMG, ENG, ERG). <http://www.nlm.nih.gov/medlineplus>.
Bryan Ronain Smith